The research aims to describe in molecelar terms the mechanism of absorption of calcium acros the small intestine and the role therein of vitamin D and related substances. The absorption involves an active cation pump for calcium comprising at least 2 steps, entry into the mucosal cell and energy-coupled exit at the serosal face. Vitamin D is associated with the maintenance of protein elements which may be components of the calcium pump. These include a soluble calcium binding protein, a brush border Ca-dependent ATPase, and other components under investigation. These tissue elemets correlate with calcium transport in the rat. The role of these elements in transport of calcium and other divalent cations (Zn, Fe, Mn, Cd) is under investigation.